Food cooking oven with sterilization control arrangement

ABSTRACT

Food cooking oven, comprising a thermometric probe adapted to be introduced in the food to be cooked and to generate a signal that is representative of the detected temperature, in which said signal is sent to appropriate processing and display means capable of working out a final information (F) depending in a combined manner on both the detected temperatures and the cooking time. The oven is further provided with selection means adapted to send selective commands to said processing means. 
     These processing means are furthermore adapted to automatically perform a comparison of said final information with said selective commands and issue respective signals, corresponding to the outcome of said comparison, towards appropriate indicator means. These signals are indicative of the period of time during which the food can be kept stored, ie. preserved after cooking, since they represent the bacterial count at the end of the cooking process.

This application claims the benefit of International Application No.PCT/EP01/08233, which was published in English on Mar. 21, 2002.

DESCRIPTION

The present invention refers to a food cooking oven, in particular ofthe kind intended for use in catering and foodservice applications,provided with special features adapted to inform the operator, ie. theuser about the bacterial count persisting in the food being handled, andin particular to give an information about the period of time duringwhich the just cooked food can be kept stored, ie. can be preserved.

Food cooking oven are generally known to operate by transferring heatfrom the outside of the food being cooked to the inside thereof, andthis circumstance unavoidably leads to the fact that the food itself iscaused to undergo a differing heat treatment: in particular, theinnermost portion of the food is capable of reaching the appropriatecooking temperature within a definitely longer time and, conclusively,this innermost portion generally is cooked to a lesser extent.

This phenomenon, which by the way is largely known in the art, is ofparticular relevance and significance in connection with those kinds offoodstuffs that come in the form of a compact mass with an outwardfacing surface that is rather small as compared wit the inner volume ofthe food; these kind of foodstuffs are mainly represented by wholepieces of meat, rolled meat pieces, pieces of minced meat such as meatloaves and the like.

In view of doing away with such a typical phenomenon of anon-homogeneous cooking effect or a poor cooking effect in the interiorof the food being handled, it is a generally known practice in the artto provide some kind of thermometric probes, such as in particular thewidely used pin-type core temperature probes, which are introduced inthe interior of the food mass being cooked. The temperature informationthat is delivered by such probes is then used to control the cookingcycle, as an alternative to the cooking cycles that are on the contraryprogrammed according to the cooking time.

However, even such a use of core temperature probes is not sufficient inview of solving the problem of delivering a correct information aboutthe risk that a food, although having been cooked, may still retain anunacceptably elevated bacterial count.

It is in fact a largely known fact that the residual bacterial contentin a cooked food depends to a substantial extent on the actual timeduring which a given minimum temperature level is allowed to persist inthe same food, ie. on the same food being allowed to remain at a minimumtemperature level for a definite period of time, and such an informationis not automatically and readily available in prior-art cooking ovens.As a matter of fact, in prior-art ovens the temperature is detectedeither in a continuous manner or at pre-determined intervals during thecooking cycle, and it is only at the end of such a cooking cycle that askilled operator is able to judge whether the residual bacterial contentin the cooked food is at an acceptable level or not.

However, such an information can solely be inferred, ie. is solelyavailable if the operator of the oven is adequately trained to do suchkinds of assessments, and even in his case only at the end of thecooking process, so that if the residual bacterial content is judged tobe still in excess of an acceptable level, owing to the cooking cyclebeing already concluded, potentially dangerous conditions of uncertaintymay well arise.

Furthermore, no automatic indication is given or available about theactual length of the period of time during which the just cooked foodmay be kept in store, ie. preserved before the bacterial content islikely to rise again to an unacceptable level. Last, but not least, if athermometric core temperature probe inserted in the food to be cooked isused to control the cooking cycle, there is no certainty at all that thethereby measured temperature is actually the temperature prevailing inthe coldest point within the food.

For instance, said solution is divulged form WO 01/73352, illustratingan invention that concerns a cooking device comprising a chamber forfood provided with heating means with controlled operation, and room andinternal temperature sensors, means for acquiring internal and roomtemperatures and a minimal decontamination value and a control modulefor monitoring the heating means in accordance with parameters based onthe internal and room temperatures input and the decontamination valueinput; however said solution presents the above cited drawback that themeasured temperature is actually the temperature prevailing in thecoldest point in the food, and furthermore the control of the heatingmeans based also on the cooking chamber temperature cannot assure thatthe temperature inside the food will rise at a pre-defined level for agiven lenght of time.

From EP 0 794 387 a method for estimating the temperature of the innerportion of material to be cooked and thermal cooking apparatus using thesame method are divulged; however said document does not teach how tosolve the problems due to the detection of the actual lower temperatureinside the food, and moreover the proposed method appears complicate andsomething unreliable due to the need that the physical properties of thefood have to be learned and entered in the relevant oven.

A further basic element of uncertainty and, therefore, of potentialdangerousness is due to the fact that the various foodstuffs have widelydiffering properties, ie. behave in a totally different manner in thisconnection, so that, as all those skilled in the art are well aware of,the bacterial count of a certain food that has gone through a standardcooking cycle, and that for this reason may then feature a significantlyreduced bacterial content, can on the contrary turn out to be stillunacceptably elevated in another kind of food that is treated using thesame cooking cycle.

Finally, a further element that is placing pressure on the manufacturersof foodservice equipment in general, including food cooking ovens, isthe European Directive no. 93/43 (the so-called HACCP-Directive, wherethe acronym stands for Hazardous Analysis and Critical Control Point),which is aimed at introducing hygiene, health-safeguard and safetyrequirements in food processing, cooking and storage equipment andprocesses.

The need is therefore strongly felt for providing a food cooking oven,particularly intended for foodservice and catering applications, whichallows for the above mentioned drawbacks and risk situations to be rightaway eliminated or at least reduced to acceptable levels, and whoseconstruction is not only simple, but makes also use of readily availabletechniques. Furthermore, the utilization of such a kind of oven shall besimple and filly within the capability of a normally skilled operator offood cooking ovens, in particular food cooking ovens used in foodserviceand catering applications.

These aims, along with further features of the present invention arereached in a food cooking oven that is made and operates in accordancewith the characteristics that are recited in the appended claims.

The present invention may be implemented according to a preferred,although not sole embodiment that is described in detail and illustratedbelow by way of non-limiting example with reference to the accompanyingdrawings, in which:

FIG. 1 is a symbolical view of the control and display panel of a foodcooking oven according to the present invention;

FIG. 2 is a side view of a pin-like core temperature probe according tothe present invention;

FIG. 3 is a diagrammatical view charting, in the same graphrepresentation, the evolution pattern of both the core temperature in afood and the sterilizing effect F (as defined further on) as a functionof the cooking time;

FIG. 4 is a logic flow chart of the operation of a food cooking ovenaccording to the present invention;

FIG. 5 is a view in table form of a representative classification of aplurality of different kinds of food according to HACCP criteria.

The present invention is based essentially on following considerations:since the bacterial content in a food is generally known to vary as afunction of both the temperature to which the food itself is exposed andthe length of time of such exposure to said temperature, the possibilityis given for a function to be plotted that links the reduction in thebacterial content with both these parameters and that, as a result, isrepresentative of the evolution pattern of the same bacterial content inthe food.

Such a function is commonly known as the function F, which is definedas:

the measure of the sterilizing effect F that is reached in the cookingprocess (a function of the kind of food product being handled and themicro-organisms that most probably are present in it):

F=D _(To)(LogN _(o)−LogN)=nD _(To)  (1)

where:

F=sterilizing effect or mortality rate.

D_(To)=decimal reduction or decay time for the micro-organism taken as areference: it represents the length of time of exposure to the constanttemperature T_(o) that is necessary for the concentration of vital cellsto be reduced by 10 times, ie. the length of time needed to attain theinactivation of 90% of the initially present cells or spores.

T_(o)=reference temperature (eg. 71° C. for pasteurization).

N_(o)=initial microbial concentration.

N=final microbial concentration.

n=number of resulting decimal reductions.

Having therefore so defined the function of bacterial content reduction,the possibility arises for a plurality of degrees of known reductionthereof to be defined as well, which correspond to respective valuesF_(o), F₁, F₂ . . . F_(n) that such a function F can take.

As this will be exemplified further on, to these values there can beassociated pre-established periods of preservability under storageconditions (ie. shelf life) of the food items having been processed.

In a few words, use is made of a conventionally established andacknowledged function of the evolution pattern of the bacterial count,ie. content in a food, and some characteristics of the hygienic state ofthe same food are identified experimentally along with the aptitudethereof to be kept in store, ie. preserved before its bacterial contentrises again to an unacceptable level.

To these characteristics there are then associated a plurality ofcorresponding values which said function F may attain. It is quiteapparent at this point that, if the evolution pattern of such a functionF during a cooking cycle is plotted in a substantially continuousmanner, it is possible for the moment to be detected, ie. identified, atwhich such a function successively reaches the various afore determinedvalues of F. In this way, it is therefore possible to be continuouslyinformed on the variation of the bacterial content of the food beingcooked and, in short, the cooking cycle itself may be determined also bya suitable comparison of the values that are so successively reached bysaid function F with the corresponding pre-determined values.

Furthermore, in view of making the present invention still moreinteresting and adhering to the different reality of the variousfoodstuffs to be cooked, this method can be appropriately fine-tuned byprogramming the oven, or rather appropriate processing devices thatcontrol and adjust the oven, with the input of an information concerningthe nature of the food being cooked. In other words, through anappropriate control means, which will be duly dealt with further on inthis description, two or more categories of micro-biologicaldangerousness associated to the foodstuffs to be cooked can be entered.

Each such category of dangerousness is characterized by a differentinitial microbial concentration N_(o), as well as different values ofD_(To) and z.

As a result, in correspondence of preestablished values of finalmicrobial concentration N that can be assumed as being correlated withdifferent shelf-life values, ie. different storability periods, thereare to be found different values of F_(o), F₁, F₂ . . . F_(n), whichvary in accordance with the nature of the food being handled.

F is calculated by integrating the temperature T in the length of time tthrough the following relation derived from an elaboration of (1):F = ∫_(t₁)^(t₂)10^((T − To)/z) ⋅ t

where:

T=temperature at the core of the food (function of time)

t₁=instant to which there corresponds a temperature in excess of aspecified value (eg. 50° C.)

t₂=instant of final reading

z=temperature increment with respect to T_(o) bringing about a decimalreduction D_(to) (function of the heat resistance of the individualmicro-organisms) and is therefore to be considered as a constantcharacteristics of the particular kind of food.

With reference to FIG. 3, this can be noticed to plot, with the curveindicated at R, the evolution pattern of the core temperature, ie. thetemperature at the innermost portion, of a typical piece of chicken,while the curve indicated at S in the same Figure illustrates theevolution pattern of the afore defined function F.

From this graph it can be noticed that the function F shows a sharplyincreasing evolution pattern starting from a minimum time of approx. 35minutes, and furthermore that to a temperature of 90° C. therecorresponds a function F of more than 460 and this datum will actuallybe used in the example appearing further on in this description.

With reference to FIG. 1, it should be noticed that the control anddisplay panel 1 includes a thermostat switch 2, a timer switch 3 and,possibly, a cycle selector switch 4, wherein all such means arewell-known and generally used in the art.

Anyway, the control and display panel according to the present inventionfurther includes:

a plurality of selector means 5, 6 (appearing in the form ofpush-buttons in the Figure)),

a visual display 7 adapted to bring out information on the state of thereduction in the bacterial content in relation to the on-going cookingprocess.

It is also a largely known fact that foodstuffs of different natureusually feature different characteristics as far as the reduction inbacterial content as a function of cooking temperature and cooking timeis concerned. In view of more effectively exemplifying such acircumstance, FIG. 5 illustrates a table in which, under the headingHACCP Classification, there appear some examples of both high-risk andmedium-risk foodstuffs from a bacteriological point of view, along withrespective rating codes A and B.

EXAMPLE Food: Chicken

N_(o) (initial microbial concentration)=10⁴ cells/g

N (final microbial concentration for consumption, ie. eating within 5hours)=10² cells/g

T_(o)=71° C. (pasteurisation temperature).

Reference Micro-organism:

Lysteria monocytogenes >>D₇₁=0.23 minutes, z=10

t₁: time corresponding to core temperature T_(core)=50° C.

t₂: end-of-cooking time.

Calculation of the safety value F_(o) required in view of attaining afinal microbial concentration N:

F _(o) =D _(To)(LogN _(o)−LogN)=0.23 (Log10⁴−Log10²)=0.46 minutes

which in other words means that the required sanitizing effect can beattained by means of a heat treatment process whose effectiveness isequivalent to a permanence for a time of 0.46 minutes at a constanttemperature of 71° C.

Since the formula for the real-time calculation of the actual value of Fis: F = ∫_(t₁)^(t₂)10^((T − 71)/10) ⋅ t

the required safety condition is represented by:

F>F_(o)

During cooking, the value of F is calculated in a continuous manner, andis further compared with values F_(o), F₁, F₂ . . . F_(n) that arecontained in a pre-defined table, in which said values are associated toand characteristic of the kind or category of the food being each timehandled.

In the above example and the associated FIG. 3, at the end of thecooking process, ie. after approx. 45 minutes, there results F=460, sothat the safety condition requirement appears to be largely compliedwith, owing to a reduction in the bacterial content of

n=F/F _(o)=460/0 46=1000

having been obtained, ie. a bacterial content reduction by 1000 times.

At this point, all those skilled in the art should be fully capable ofclearly understand how an oven according to the present inventionactually operates: in fact, the oven operator selects one of thecategories to which the food to be cooked belongs (eg. A or B) andenters such information through the proper selector means 5, 6 (whereinit shall of course be appreciated that this non-limiting example doesnot exclude the possibility for the food categories to be more than two,with said selector means being of course capable of entering exactly theidentification of the selected food category).

Furthermore, the operator introduces in the food a pin-like thermometriccore temperature probe 11 which is connected to appropriate decodingcircuits to deliver compatible signals that can be used by anappropriate processing and control device (not shown).

As for the rest, the cooking oven is programmed in either atime-controlled or a temperature-controlled mode, in a largelytraditional manner.

At the beginning of the cooking process, the processing and controldevice at the same time starts calculating the integral of the formula(1) in view of delivering at each single instant the value of F.

At each such instant, said processing and control device automaticallycompares the value of F, as this has so been just calculated, with a setof values F_(o), F₁, F₂ . . . F_(n) that will have been appropriatelypre-defined and stored in the unit's memory.

These values are determined experimentally for each category of food, Aor B, as previously selected through said selector means 5, 6, in viewof identifying the corresponding maximum allowable shelf-life, ie. thelongest storage period before the bacterial content of the foodincreases again to an unacceptable value.

For instance, a determined code, eg. “SAFE 1”, to be transmitted to saidvisual display 7, may be associated to a given value of F. The sameapplies to all other pre-determined values of F, as this is betterexemplified in the Table below:

Value of F Display Max. allowable shelf-life F < F₀ UNSAFE F >F₀ SAFE-05 hours F > F₁ SAFE-1 1 day (refrigerated storage) F > F₂ SAFE-2 5 days(refrigerated storage) . . . . . . F > F_(n) SAFE-n . . .

To the code SAFE-0 there will for instance correspond, for each selectedcategory of food, a maximum allowable time of 5 hours before said foodis eventually served for consumption; again, to the code SAFE-1 theremay correspond a shelf-life of one day if the food is kept under correctrefrigerated storage conditions.

In this way, at the end of the cooking process the operator isimmediately informed about the cooked food having a more or lessacceptable bacterial content, ie. the bacterial count thereof havingbeen reduced to a more or less acceptable value, and the actual lengthof time during which the same food can then be kept in store beforeserving, without any risk, under particular pre-established storage orambient conditions (eg. refrigeration).

A drawback may therefore arise in that, even upon the conclusion of aregularly performed cooking process, the value of F, ie. F_(meas), comesto lie below a minimum pre-established value, ie. a value F_(o) to whichthere corresponds the minimum shelf-life threshold value of five hours.

An advantageous improvement of the present invention then consists inadopting particular comparison, processing and control criteria andmodes, such that, under the above cited circumstance (ie.F_(meas)<F_(o)), the cooking cycle is able to go on automatically atleast until said value of F eventually reaches the value of F_(o).

It will of course be appreciated that the above method may beadvantageously accompanied by further devices and arrangements that arefor instance capable of informing the operator, eg. through visualand/or sound indications, that the cooking process is automaticallycaused to go further on for a period of time as necessary for the abovecited condition F_(meas)<F_(o) to be attained.

A further improvement of the present invention consists in implementingsaid pin-like thermometric core temperature probe 11 in such a manner asto provide the same probe with a plurality of distinct points 20, 21, .. . 25, each one of which is then capable of measuring the respectivetemperature and sending the corresponding information to said device 10.

This improvement practically ensures that, if the pin-like thermostaticprobe is arranged in an inappropriate, ie. not so correct manner withinthe food to be cooked, the availability of more temperature probingpoints in it enables a number of respective, generally differenttemperature values to be sent to said processing device, among which thelowest one can then be identified.

Said lowest temperature value represents the temperature of the pointthat most probably is the coldest spot in the food being cooked, and cantherefore be automatically selected as the temperature on which can thenbe based the calculation of the value of F that will of course beselected for carrying out the process in the most appropriate manner inaccordance with the above illustrated principles.

What is claimed is:
 1. A method of running a food cooking oven, intendedin particular for use in foodservice and catering applications,comprising the introduction of a probe in the interior of the food beingcooked and generating an electric signal that is representative of thedetected temperature, in which said signal is sent to an appropriateprocessing and control device, characterized in that said processing andcontrol device is adapted to work out an information (F) that depends ina combined manner on both the temperatures detected by said probe andthe cooking time, and that said information is a value that isrepresentative of the reduction in the bacterial content of the cookedfood, and that said processing and control device selects, for thereal-time calculation of said information (F), the lowest temperaturebeing detected by a plurality of temperature sensors located atdifferent points (20, 21, . . . 25) of said probe that is preferably apin probe (11).
 2. A method for running a food cooking oven according toclaim 1, characterized in that it is further provided with selectormeans (5, 6) that are adapted to classify the food being cookedaccording to predefined categories (A, B) and to send a multiplicity ofpre-defined values (F_(o), F₁, F₂ . . . F_(n)), depending on theselected category, towards said processing and control device, which isin turn adapted to perform a comparison of such worked-out value of Fwith said values (F_(o), F₁, F₂ . . . F_(n)) and to issue respectivesignals (UNSAFE, SAFE-0, SAFE-1, . . . ), corresponding, to the outcomeof said comparison, towards appropriate indicator or display means (7).3. A method for running a food cooking oven according to claim 2,characterized in that, if said comparison identifies a value of saidinformation being lower than a pre-determined limit (F_(o)), a specificwarning signal (UNSAFE) is automatically generated and issued fordisplay (7).
 4. A method for running a food cooking oven according toclaim 3, characterized in that, if at the end of the cooking processsaid comparison gives a result of said value (F) being lower than apre-determined limit (F_(o)), the cooking process itself isautomatically caused to go further on until the outcome of saidcomparison eventually attains at least said pre-defined value (F_(o)),in which case the visual message on the display is updated accordingly(UNSAFE>SAFE₀).
 5. A method for running a food cooking oven according toclaim 2 or 3 or 4, characterized in that said respective signals(UNSAFE, SAFE-0, SAFE-1, SAFE-2 . . . ) are representative of the lengthof time during which said cooked food may then be kept in store beforebeing served after cooking.
 6. A method for running a food cooking ovenaccording to any of the preceding claims, characterized in that saidprocessing, ie. calculation is performed through the integration of thefunction that is representative of a temperature of the food beingcooked.
 7. A method for running a food cooking oven according to claim1, characterized in that said probe is a pin-like core temperature probe(11) provided with a plurality of temperature sensors located atdifferent points (20, 21, . . . 25), and that said processing andcontrol device selects, for the real-time calculation of saidinformation (F), the lowest temperature being detected by said pluralityof temperature sensors.